Yeah, love triangle, Kirishima like Mega, Mega likes both Reiko and Kirishima.
Since Mega can't choose, Reiko bullies Kirishima using Yuuko rumour, it's not really Yuuko fault it's those who spread rumours.
Reiko driven by guilt and trying to attone for her bullying attempt to sacrifice her life or healthy by jumping off.
Since Kirishima somehow doesn't mind her being bullied and Reiko apologies, forgives her and dumps all fault on Yuuko. It's not her fault, those were stupid people rumours.
Seems like Yuuko created her evil self willingly, pushed all memories and resentment to her. What's holding her from merging is her feelings for Teiichi.
Thanks for scanlation!